The present invention relates to hydraulic control systems, and particularly to systems in which a pair of hydraulic operators can be driven either in parallel or in series to power a common load.
Construction equipment such as hoists have moveable members that are driven by a hydraulic operator, such as a hydraulicly powered motor or a cylinder/piston arrangement. Application of hydraulic fluid to the operator traditionally was controlled by a manually operated valve, such as the one described in U.S. Pat. No. 5,579,642. This type of valve had a manual operator lever mechanically connected to a spool which could slide within a bore of the valve body. The pump and tank lines of the hydraulic system connected to ports of the valve body and the operator was coupled to workports on that valve body. Movement of the spool into various positions with respect to cavities in the bore enabled pressurized hydraulic fluid to flow from the pump to the operator and return to the tank also through the valve.
Manual valves are required to be mounted in the operator cab of the equipment thus requiring that a pair of hydraulic lines be run from each valve to the associated operator. There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located in the operator cab. Instead the solenoid valves are mounted adjacent the operator, thereby requiring that only a common hydraulic line be run from the pump and a common return line be run back to the fluid tank. The solenoid valves distributed throughout the equipment connect to this single pair of hydraulic lines. Electrical controls are mounted in the cab with wires running to the respective solenoid valves. Wires are easier to run throughout the equipment and are less prone to failure than pressurized hydraulic lines.
Some hydraulic applications utilize a pair of operators to power a common load and power those operators in parallel or in series in different operating modes. For example, lift hoists utilize a pair of hydraulic motors to drive the cable spool which raises or lowers a load. The motors usually are often connected in parallel for greater power to lift heavy loads. The motors are connected in series to lower the load permitting increased speed of the cable spool when less power is required as gravity aids that lowering. In this application, the two operators typically are connected to a four-way spool valve and a series-parallel circuit that changes mode as commanded by operation of the spool valve. Such systems require two valve housings and intricate valving.